1. Field of the Invention
The present invention relates to an improved bullet tube expanding apparatus and an improved bullet assembly for use in such an apparatus.
2. Disclosure Information
A bullet tube expanding apparatus is a well known apparatus which has been developed for expanding tubes by passing a mandrel or bullet mounted upon an elongate stem down the tube through the tube to expand the diameter. The diameter of the bullet is large enough to mechanically expand the tube beyond its elastic limit so that it retains the larger expanded diameter. The typical use is for expanding tubes to be secured to fins and end sheets to form heat exchangers.
In such an application a bank of bullets is passed through the tubes of a loosely assembled heat exchanger matrix until the whole length of the each tube is expanded causing an interference fit between the outsides of the tubes and the insides of the apertures of the fins. Typically the diameter of each such tube will be in the region of 6 mm to 8 mm.
The tool which is passed through the tubes is typically of steel and is known as the bullet. In the majority of applications the bullet expands the tube when it is pushed into the tube and the stem of the bullet is in compression. This means that there can be risk of buckling under load.